In this paper we present a context-based family of formal systems (called MultiContext Systems) which we propose to use as a formal framework for a theory of mental representation. We start with an intuitive notion of context as a subset of the complete (cognitive) state of an individual. Then we introduce two general principles which we believe are at the core of any logic of context, namely the principles of locality and compatibility. We show how these principles can be formalized in the framework of MultiContext Systems, and argue that this conceptual/logical framework can be used to account for a variety of phenomena in a theory of mental representation. Finally, we compare our framework with previous work.